1. Field of the Invention
The present invention relates to a color image forming device such as a copier, facsimile device, printer or the like which utilizes an electrophotographic system in which toner images of different colors respectively formed on a plurality of image carriers are superimposed and transferred onto a transfer body or recording medium.
2. Description of the Related Art
Conventionally, devices equipped with a so-called printer engine constructed from a plurality of photosensitive bodies as image carriers, an endless intermediate transfer belt as a transfer body that successively passes through transfer positions mutually facing the respective photosensitive bodies, and a conveyor belt as a conveying member which holds and conveys a recording paper as a recording medium, have been known as color image forming devices of this type. In such color image forming devices, toner images of mutually different colors formed on the surfaces of the respective photosensitive bodies by the electrophotographic process are respectively overlapped and transferred onto the surface of the intermediate transfer belt, or onto the surface of the recording paper that successively passes through the respective transfer positions while being held on the conveyor belt, so that desired color images can be obtained.
In such a color image forming device, in cases where positional deviation occurs in the transfer positions of the respective color toner images when the respective color toner images formed on the respective photosensitive bodies are transferred onto the surface of the intermediate transfer belt or recording paper, the so-called “overlap deviation phenomenon” occurs in the color images obtained by the superimposition of the respective color toner images, so that the problem of a drop in image quality arises. The transfer position deviation that causes this overlap deviation phenomenon in such color images is caused by fluctuations in the light path due to temperature variations in the optical system that optically scans the respective photosensitive bodies, variations in the linear velocity of the respective photosensitive bodies and the like.
Conventionally, therefore, detection pattern toner images are formed on the respective photosensitive bodies, these detection pattern toner images are transferred onto the intermediate transfer belt or recording paper, and the spacing of these transferred detection pattern images is measured. Then, the exposure timing is varied so that this spacing is constant, and the positional deviation of the toner images in the respective transfer positions is thus corrected.
However, in the case of a method in which the transfer position deviation of the respective transferred color toner images is thus corrected by varying the timing of the exposure performed on the respective photosensitive bodies, the minimum amount of positional deviation that can be corrected corresponds to the dot diameter of the light beam that exposes the respective photosensitive bodies, e.g., 42 μm. Accordingly, in this conventional transfer position correction method, amounts of positional deviation at dimensions smaller than the dot diameter of the light beam cannot be corrected. Accordingly, the following problem arises: namely, there are limits to the correction of the positional deviation of the respective color toner images that can be accomplished.
Furthermore, for example, in the image forming device disclosed in Japanese Patent Application Laid-Open No. 10-20607, in order to solve the problem of positional deviation of the respective color toner images, i.e., the problem of the overlap deviation phenomenon caused by this positional deviation, as described above, the occurrence of deviation in the superimposition of the respective color toner images is prevented by separately setting and processing the driving speeds of the respective photosensitive bodies. In the case of such setting and processing of the driving speeds of the respective photosensitive bodies, specified reference toner images are first formed on the respective photosensitive bodies at a specified timing, and these reference toner images are transferred to the surface of a belt member. Next, the relative amounts of positional deviation of the respective reference toner images with respect to each other are calculated on the basis of the timing at which these respective reference toner images are detected by a photo-sensor. Then, on the basis of the results of these calculations, the driving speeds of the respective photosensitive bodies are individually set, and the respective photosensitive bodies are driven at speed differentials corresponding to the respective amounts of positional deviation. As a result, deviation of the transfer positions of the toner images on the respective photosensitive bodies can be prevented, so that the respective color toner images can be effectively superimposed.
Meanwhile, the variation in the linear velocity of the photosensitive bodies that causes the overlap deviation phenomenon of the respective color toner images described above is caused by variation in the diameter dimensions of the respective photosensitive bodies, variation in the dimensions of the respective parts constituting the driving force transmission system that applies a rotational driving force to the respective photosensitive bodies, and the like. For example, variation in the linear velocity of the photosensitive bodies caused by eccentricity of the photosensitive body gears that are fastened to the rotating shafts of the photosensitive bodies is known.
Specifically, in such photosensitive body gears, when the location where the radius is the greatest due to eccentricity engages with the gear on the driving side, the linear velocity of the photosensitive body is slowest. On the other hand, when the location where the radius is smallest engages with the gear on the driving side, the linear velocity of the photosensitive body is highest. Since the former location and the latter location in the photosensitive body gear are located in point-symmetrical positions at 180 degrees from each other with reference to the center of rotation, fluctuating characteristics that trace a sine curve of one period appear in the linear velocity of the photosensitive body for each revolution of the gear. Furthermore, since the surface of each photosensitive body passes through the transfer positions at a maximum speed when the photosensitive body is rotating at a linear velocity corresponding to the upper limit of the sine curve, the toner image is transferred onto the recording paper in a shape that is extended further in the direction of movement of the surface of the photosensitive body than would inherently be the case. On the other hand, since the surface of the photosensitive body passes through the transfer positions at a minimum speed when the photosensitive body is rotating at a linear speed corresponding to the lower limit of the sine curve, the toner image in this case is transferred onto the recording paper in a shape that is more contracted in the direction of movement of the surface of the photosensitive body than would inherently be the case.
Thus, in the transfer process in which the respective color toner images are superimposed, when toner having a shape that is more contracted than the inherent shape is transferred onto a toner image with a shape that is extended further than the inherent shape on the intermediate transfer belt or recording paper, the abovementioned overlap deviation phenomenon occurs. Furthermore, this overlap deviation phenomenon also occurs when toner having a shape that is extended further than the inherent shape is transferred onto a toner image with a shape that is more contracted than the inherent shape.
In recent years, a type of control referred to as so-called “phase alignment”, in which the rotational phases of a plurality of photosensitive body gears are synchronized, has been used in order to avoid such an overlap deviation phenomenon. Specifically, in this phase alignment control, marks are first applied to the locations of maximum diameter or locations of minimum diameter on the photosensitive body gears that are fastened to the rotating shafts of the respective photosensitive bodies, and these marks are detected by a photo-sensor or the like. Then, the rotational angles of the respective photosensitive body gears are respectively grasped on the basis of these detection results. If the respective photosensitive bodies are individually stopped at mutually identical rotational-angular positions on the basis of this understanding when the image formation operation is completed, then the rotational phases of the respective photosensitive body gears can be synchronized in the next image formation operation. Furthermore, the system may also be devised so that the rotational phases of the respective photosensitive body gears are aligned by temporarily causing the driving speeds of the driving sources of the respective photosensitive bodies to differ from each other on the basis of the amount of deviation in the rotational phase grasped by the detection results of the photo-sensor at the time that the image formation operation is initiated.
However, even if the rotational phases of the respective photosensitive body gears are aligned in this way, the painstakingly aligned rotational phases are skewed by the linear velocity difference in the respective photosensitive bodies in the case of a construction that imparts a linear velocity difference to the respective photosensitive bodies as in the image forming device disclosed in the abovementioned Japanese Patent Application Laid-Open No. 10-20607. Furthermore, as this linear velocity difference is increased, the skewing of the rotational phases of the photosensitive body gears becomes more severe, so that overlap deviation tends to be generated by phase deviation. Especially in the case of a continuous printing mode, i.e., a continuous image formation operation, in which images are continuously printed out on a plurality of recording papers, the phase deviation increases each time that an image is printed out, so that the problem of conspicuous overlap deviation arises.
Technologies relating to the present invention are (also) disclosed in, e.g., Japanese Patent Application Laid-Open No. 10-078734 and Japanese Patent Application Laid-Open No. 2001-005363.